Shrinking design rules, increasing numbers of metal interconnect layers, and increasing demand for planarization from deep ultra-violet ("DUV") lithography systems are spreading the use of Chemical Mechanical Polishing/Planarization ("CMP") processes throughout the microelectronics industry. CMP manufacturing applications range from microprocessors to Dynamic Random Access Memories ("DRAM"), flat panel displays and multi-chip modules. As more manufacturing processes benefit from planarization, the need and demand for integrated CMP processes grows.
A complete and integrated CMP production line consists of: a polisher, consumables, chemical distribution, cleaning systems, measurement, process isolation, environmental control and material handling systems. Providing an enclosure system that properly manages controls and enhances the process environment is critical because present CMP tools either do not provide enhanced process environments or fail to fully manage cross-contamination and environmental conditions that affect the process and the slurry.
Improperly managed slurry and CMP chemicals can be a contamination source to production. First, slurry or CMP chemicals can cross-contaminate other production processes. If wafers, carriers, boxes or personnel leave the CMP production area contaminated with chemicals or slurry, cross contamination of other fabrication areas may take place. Other chemicals used as part of the CMP process can cross-contaminate other production areas. Lithography processes, for example, using DUV photoresist require planarization. Ammonia, which is present in certain CMP applications, is a "killer" contaminant for ultra-sensitive DUV photoresist, neutralizing the photoresist acids in the ppb range. Second, the slurry used in wafer planarization makes CMP a "dirty" process. In aqueous solution, slurry particles are successfully managed for wafer polishing and planarization. Stagnant slurry and slurry particles that fall out of solution, however, can dry to a powder or even cement-like condition. Dried slurry may also cause wafer defects and low production yield. For instance, dried slurry particles can scratch and gouge wafer surfaces, eventually resulting in metal shorts after successive metal deposition.
Additionally, chemicals used in the CMP and post-CMP cleaning process can be hazardous to personnel. OSHA guidelines suggest that slurry, cleaning solutions and resulting chemical vapors used in CMP processes might pose a health risk to operators. For instance, some slurries and CMP-related chemicals may use potassium and sodium hydroxide. These strong alkali compounds can burn the skin and eyes on contact or, in their vapor form, cause eye and respiratory irritation. CMP processes may also use soluble and insoluble tungsten, which is linked to "hard metal disease" and may be lethal if ingested. Ammonia used in post-tungsten cleaning and ammonia released from other sources irritates eyes and the respiratory system and masks the odor of other gases.
CMP contamination issues also present logistical problems for integrating the CMP sector with other production areas that are more sensitive to contamination. A common practice is to separate physically with cleanroom walls the polisher from clean/dry input/output areas and metrology areas. Alternatively, stand alone polishers may be placed in gray clean room areas and a material handling strategy used to safely stage, transfer and transport wafers to and from other parts of the production line. In either case, personnel must access both the clean fabrication space and the gray CMP production areas. But this tool separation requires that personnel change garments between each area to prevent cross contamination of the clean areas of the fabrication. Resulting productivity losses greatly increase costs.
Managing environmental conditions also is important to maintaining the production readiness (uptime) of the tool, which effects the wafer throughput. Planarization rates, tool down time and maintenance activity all impact wafer throughput in the polishing process. Planarization rate is the rate at which material is removed from the wafer surface; it is affected by the slurry viscosity and the ability of the polishing pad to transport liquid and to remove excess slurry. Slurry viscosity, in turn, will be affected by the liquid content of the solution and environmental conditions. Environmental conditions include air flow rates, air turbulence, temperature and relative humidity. Dry production environments or ones with high air flow rates may tend to dry the slurry. Dried slurry buildup on tools and in delivery lines must be routinely cleaned. Clogged slurry delivery lines must be flushed and cleaned, requiring additional tool maintenance. Wet slurry may be easily rinsed away but dried and hardened slurry is much more difficult to remove.
Current designs do not solve all these problems. For instance, many CMP tool suppliers incorporate some type of enclosure or hood system to contain slurry and chemical vapors. The standard polisher tool configuration incorporates filtered air supply only to the input/output ("I/O") area to protect staged wafers with positive air pressure. The polishing process area may be kept at a negative pressure using facility exhaust to remove any chemical vapor contamination. In short, current designs only attempt to capture contamination and exhaust it. No provision is made to prevent ambient contamination from entering the process. No attempt is made to reduce slurry drying.
Those "standard" enclosure systems also are neither adequate for cross contamination control nor optimized for total control of the environmental conditions that effect tool availability wafer through-put and yield. Operating a CMP tool enclosure with negative pressure allows infiltration of contamination generated within the fabrication area into the process area and sends excessive and unnecessary volumes of air to the fabrication scrubber, thereby increasing operation costs. A polisher located in a gray area has an increased chance for contamination from the degraded environment. Macroscopic defects are typically caused by either large particles that enter the polishing chamber or large deposits of dried slurry that fall back onto a wafer in the CMP process chamber. In addition, the lack of air flow management can result in adverse air flow conditions over the polishing pads. Excessive air turbulence can lead to slurry contamination problems.